Development of standards for conveying high dynamic range (HDR) and wide colour gamut (WCG) video data and development of displays capable of displaying HDR video data is underway. Standards bodies such as International Organisations for Standardisation/International Electrotechnical Commission Joint Technical Committee 1/Subcommittee 29/Working Group 11 (ISO/IEC JTC1/SC29/WG11), also known as the Moving Picture Experts Group (MPEG), the International Telecommunications Union-Radiocommunication Sector (ITU-R), the International Telecommunications Union-Telecommunication Sector (ITU-T), and the Society of Motion Picture Television Experts (SMPTE) are investigating the development of standards for representation and coding of HDR video data.
HDR video data covers a wide range of luminance intensities, far beyond that used in traditional standard dynamic range (SDR) services. For example, the Perceptual Quantizer (PQ) Electro-Optical Transfer Function (EOTF), standardised as SMPTE ST.2084, is defined to support a peak luminance of up to 10,000 candela/metre2 (nits) whereas traditional television services are defined with a 100 nit peak brightness (although more modern sets increase the peak brightness beyond this). The minimum supported luminance is zero nits, but for the purposes of calculating the dynamic range the lowest non-zero luminance is used (i.e. 4*10−5 nits for PQ quantised to 10 bits). The physical intensity of a light source is measured in candela/meter2 and is also referred to as ‘luminance’ or ‘linear light’. When luminance is encoded using PQ (or other transfer function) the encoded space is referred to as luma′. Luma is intended to be more perceptually uniform (i.e. a given change in the luma value results in the same perceived change in brightness regardless of the starting point). Traditional power functions such as the ‘gamma’ of SDR television is somewhat perceptually uniform. Transfer functions such as PQ are designed according to models of human visual perception to be more perceptually uniform. In any case, the relationship between luma and luminance is highly non-linear.
Video data generally includes three colour components, where each frame comprises three planes of samples and each plane corresponds to one colour component. The relationship between the sampling rates of the planes is known as a ‘chroma format’. When each plane is sampled at the same rate, the video data is said to be in a ‘4:4:4’ chroma format. In the 4:4:4 chroma format, each triplet of collocated samples forms a ‘pixel’, having a colour and luminance resulting from the values of the triplet of collocated samples. When referring to a sample to which a gamma-correction or a transfer function was already applied, the colour component is referred to as ‘chroma’ and the luminance component is referred to as luma′ to reflect the fact that the colour components' values are not ‘true’ colour and luminance. The prime symbol (′) is sometimes used after the variable name to indicate a luma value (e.g. Y′). When the second and third of the three planes is sampled at half the rate horizontally and vertically compared to the first plane, the video data is said to be in a ‘4:2:0’ chroma format. As the use of the 4:2:0 results in fewer samples being processed compared to 4:4:4, the result is lower complexity in the video codec. Then, each pixel has one luma sample and groups of four pixels share a pair of chroma samples. Moreover, in such a case, typically the ‘YCbCr’ colour space is used, with the luma (Y) channel stored in the first plane, where the sampling rate is highest and the chroma channels (Cb and Cr) stored in the second and third planes respectively, where the lower sampling rate for chroma information results in lower data rate with little impact subjectively for viewers of the decoded video data.
When displaying the video data, a conversion back to 4:4:4 is required to map the video data onto modern display technology, such as an LCD panel. As such, a pair of chroma samples (i.e Cb and Cr samples) is combined with four luma (Y) samples. Any residual luminance information present in the Cb and Cr samples is known to interfere with the luminance information present in each Y sample, resulting in shifts in the 4:4:4 output from the 4:2:0 to 4:4:4 conversion process. In earlier ‘standard dynamic range’ (SDR) systems using a transfer function that is a power function for encoding of luma and chroma samples (i.e. a ‘gamma function’) the nonlinearity of the transfer function was less than is the case for Perceptual Quantizer (PQ) Electro-Optical Transfer Function (EOTF).